Top Voted Answer

The new System that Valve has incorporated is completely random. It means you could be playing the game normally and would get a message saying you have "found" an Item. You don't actually pick it up. You get it automatically.

All you can do is play normally and hope to find the item.

Valve have commented that the "finding" rate is a bit too low, and people playing for a few hours should find a few items. So expect a quick patch soon fixing this and giving you better chances.